A typical radio frequency identification (RFID) tag has a memory configured to store data. The data stored in the tag memory typically includes data that uniquely identifies the tag among a plurality of RFID tags. An RFID reader can be used to communicate with the tag over a radio link. Thus, the identity of the tag and the object labeled by the tag can be determined by the RFID reader in an automated process.
In a typical RFID system, an RFID reader is configured to interrogate the tags via radio frequency electromagnetic waves. The RFID reader broadcasts commands using a beam of electromagnetic wave. In response to the interrogation signals from the reader, the RFID tag may be selected to produce a radio frequency response signal.
An RFID tag may be a passive tag, a semi-passive tag, or an active tag, depending on the power source of the RFID tag and the way a response signal is produced.
A passive tag does not have an internal battery or power source. A passive RFID tag operates using the power drawn from the interrogating electromagnetic wave. A passive tag provides the response through the modulation of backscattering of the interrogating electromagnetic wave.
A semi-active tag has an internal battery or power source. A semi-active RFID tag operates using the power drawn from the internal battery or power source. A semi-active provides the response through the modulation of the backscattering of the interrogating electromagnetic wave.
An active tag that has an internal battery or power source, using which a separate transmission signal is generated to provide the response. The response signal is generated independent from the interrogating electromagnetic wave.
Radio frequency identification (RFID) tags are used in a variety of applications, such as tagging vehicles on toll roads, tagging shipping containers, quality control on assembly line conveyor belts, and monitoring tactical military equipment maneuvers, etc.
There are various ways to configure and use antennas in a. RFID system. For example, Tuttle disclosed in U.S. Pat. No. 5,572,226 an RFID system in which a plurality of antennas arranged in a two dimensional plane are combined to represent a nearly spherical antenna pattern in three dimensions. Tuttle disclosed in U.S. Pat. No. 6,574,454 an RFID system in which an antenna is coupled to a receiver via a Schottky diode. The disclosures of the above mentioned U.S. patents by Tuttle are incorporated herein by reference.